Properties for Injection Moulding
The characteristics and properties of polypropylene resins are important to a smooth and effective molding or extrusion operation and successful product performance. In order to assist customers, some basic information describing polypropylene is explained, along with a description of PP physical properties and their impact on production.
The properties of PP are influenced by a number of parameters, one of the most important is their basic form and molecular structure.
PP resins are defined as being semi-crystalline. Some of the molecular chains are able to pack relatively closely together and this leads to the formation of a limited amount of highly ordered crystalline areas.
In comparison, the molecular structures of certain other polymers commonly converted by injection molding or extrusion are much less ordered and as a consequence, they tend not to form crystalline areas. This group of polymers are defined as being amorphous.
With the basic structure of polypropylene, three configurations are theoretically possible:
Where side groups are all on the same side of the polymer chain or backbone (isotactic)
Where side groups are on alternate sides (syndiotactic)
Where side groups are distributed at random (atactic)
Commercial grades usually contain around 97% of the isotactic variant, together with 3% of atactic and virtually none of the syndiotactic variant.
These figures can be varied somewhat by altering certain process parameters during the manufacture of the polypropylene, or alternatively by changing the catalyst.
However, since each configuration tends to exhibit its own balance of characteristic properties, the ratio to each other is important. With the exception of highly specialized materials, the above ratio is generally recognized as being typical for most of the grades available today.
The properties of polypropylene and the physical properties of the finished parts are controlled by a number of basic parameters:
Basic types of PP
There are three basic PP types: Homopolymers (HOMO), Heterophasic Copolymers (HECO) and Random Copolymers (RACO). A comparison of selected properties is presented in the table below which indicates that Homopolymers have the best rigidity, Heterophasic Copolymers have the best impact resistance and Random Copolymers have the best transparency.
Generally, the 50-55% crystalline material typically seen in regular PP is midway between those in low density and high density polyethylene. Also, the size and shape of individual crystals is influenced by the thermal history to which the PP has been subjected during moulding and the presence (or not) of a nucleating agent.
In addition, HMC Polymers has for some years been able to produce special polypropylene resins exhibiting a higher level of crystallinity.
Crystal Structure: (Left) Growth pattern from nucleus (Middle) Rapidly cooled PP (Right) Slow cooled PP
When polypropylene is subjected to rapid cooling, particularly in thin sections, a multitude of crystals start to grow simultaneously. This very high rate of initiation leads to the limited amount of crystallisable material being used-up rather quickly and consequently the individual crystals remain only small in size. A similar effect is seen in the presence of a nucleant.
In comparison, when molten polypropylene is subjected to slow cooling, the rate of initiation is significantly reduced and although the amount a crystallizable material is roughly the same as before, each crystal now has less competition from its neighbors.
Consequently, the crystals are larger. The crystal structure resulting from slow cooling, can however be associated with brittleness. This is related to a tendency for the formation of micro-cracks around their boundaries when subjected to stress.
When semi-crystalline polypropylene grades are subjected to shear as they cool, crystallization may sometimes occur prematurely. The onset of shear-induced crystallization will often result in brittleness where none would normally be expected.
Polypropylene resins containing higher than normal levels of crystallinity (mentioned above) are sold under the trade name Adstif, and exhibit very high rigidity, similar to those of some of the toughened polystyrenes and PVCs.
In addition, components produced from these materials have been demonstrated by customers to have improved gloss and surface hardness, coupled with better resistance to high temperatures. They are available in all three PP types – homopolymers, random copolymers and heterophasic copolymers.
Melt flow rate (MFR)
The size and length of the growing chains are routinely manipulated during the various polymerization processes and in this way a range of grades exhibiting differing molecular masses are produced and have an affect on flow in the mould. This is expressed as the melt flow rate (MFR).
Changes to the melt flow rate have implications for both the conversion interface and for end-use performance. As the MFR is increased and flow in the mold is enhanced, both the primary and the secondary injection pressures become more efficient.
In addition, mould packing is also enhanced and so overall levels of shrinkage are reduced. In terms of physical and end-use performance properties, rigidity increases with increasing MFR, but abuse resistance and impact strength decline.
In addition, as the MFR is increased and the molecular chains become shorter, chains adjacent to each other can crystallize more readily. Resistance to creep (as distinct from creep itself) increases with increasing MFR and reducing molecular mass.
Molecular mass distribution (MMD)
The manipulation of the growing chains mentioned does not result in grades exhibiting a single value for the molecular mass, but rather a range of values that are distributed about an average.
Regular Ziegler–Natta polypropylene resins usually have a medium to broad width of distribution and the majority of grades available on the market are of this type.
However, the width may be changed during the production of the PP and in particular, there are a number of important advantages to be gained from its reduction.
Consequently, individual grades of polypropylene should be further characterised by reference to this distribution - either regular (as in the majority of cases), narrow (also known as controlled rheology or CR grades) or broad (using a latest generation catalyst or a specific reactor setup).
The width of the distribution affects both moulding and end-use performance. At an equivalent melt flow rate, as the width is reduced, the response to shear in the mould (shear sensitivity) declines and as a result, these grades flow slightly less effectively.
On the other hand, they exhibit enhanced resistance to warpage and at the same time their impact resistance tends to improve, but rigidity declines.
However, it should be stressed that all of these effects are relative and only really become significant when the distribution is particularly narrow.
More significantly, the molecular mass distribution is also affected by the nature of the catalyst used during polymerisation.
Presence of selected additives
The addition of additives such antioxidants and stabilisers, nucleants, clarifiers and antistatic agents during manufacture have an impact on the physical properties of the product.
In addition, these additives can alternatively be added later in situ by the converter, for example by the use of a masterbatch. However, dependent on the conversion process, the resultant dispersion of the additive may not be as good as that achieved by the above “fully-formulated” route and so some of the potential advantage may not be realized. This caveat is particularly relevant to high-speed injection molding.
Antioxidants and stabilizers
As a family of polymers the polypropylene resins perform best when antioxidants and stabilizers are incorporated into their formulations in order to protect them against thermal degradation during conversion as well as permitting a long service life. This is a procedure that has been established ever since their invention in the 1950s and which fully complies with the requirements set-out by the various regulatory bodies.
Normally, both the antioxidants and stabilisers are present in any additive package, due to the different and synergistic role played by them both.
Nucleants and clarifiers
In addition to resulting in the smaller crystal size mentioned above, the addition of a nucleating agent causes a significant increase in rigidity and this change can be translated into improved top-load performance.
Images courtesy of Milliken
Many homopolymer and heterophasic copolymer grades contain such additives which has particular implications for the injection moulding sector, bringing the possibility for reduced cooling times and increased overall outputs.
However, some care may need to be exercised since increased rates of set-up can sometimes lead to a reduction in the efficiency of the after-pressure, leading to slightly increased shrinkage.
The presence of dust and dirt on the surface of any injection moulded article is undesirable, especially in the case of rigid packaging. Antistatic agents are designed to limit this tendency to attract dust and dirt.
They work by migrating to the surface and then forming a cohesive molecular layer. In many instances this layer is hydrophilic and so it attracts moisture from the air and it is this that acts as an electrostatically conductive medium and helps to dissipate any static charges, thereby limiting the formation of dust patterns.
However, exceptionally low relative humidity in the conversion area (e.g. via air conditioning) may limit the efficiency of the antistatic agent.
HMC Polymers' PP grades used by customers for injection moulding are highly resistant to chemicals, essentially due to their hydrocarbon character and structure. Strong oxidising acids have been reported to chemically attack, whilst some other substances may cause swelling, but most have little or no affect.
It is strongly recommended that expert advice is always sought before polypropylene is subjected to any chemical environment. read more
Chemical Resistance PDF download
Chemical resistance data presented for a range of over 260 chemicals and substances is based on ASTM D543.
Stress cracking resistance
After exposure to certain chemical media, some polymers may form cracks when subjected to internal and external stresses below the yield-point and suffer brittle failure.
This is the phenomenon of stress-cracking, observed particularly in the presence of soaps and detergents and made worse at elevated temperatures. Unlike polyethylene, with a few well documented exceptions, PP has a good stress cracking resistance.
It is generally recognized that the barrier properties exhibited by PP to the migration of water vapour/moisture are among the best of the common commodity thermoplastics.
Resistance to the migration of oxygen is not quite as good, but our customers report that it is quite adequate to meet the requirements associated with the packaging of products with only limited shelf-lives, such as yellow fats, yoghurts and creams.
To a limited degree, PP resins are permeable to certain gasses and vapors, such as oxygen, carbon dioxide and moisture. The efficiency of a molded PP component as a barrier to the migration of these materials is thought to be related to the mobility of the molecular chains at the high-end of the distribution and in particular to the presence or absence of relatively low molecular weigh/mass chains through which they can move.
The theory suggests that this is the reason why those PP resins with a reduced amount of low molecular weight/mass material exhibit slightly superior barrier characteristics. Nevertheless, HMC Polymers' regular Ziegler-Natta Moplen types already have a proven track record.
HMC Polymers is very conscious of the need to ensure that our PP used in food packaging does not impart any taint or odor. As a result, the processes for food packaging products, together with the nature and type of any additives used are carefully screened. Once again, HMC Polymers PP grades have an established track record.
The sharpness of any notch is well-known to exert a strong influence on impact performance and a more reliable measure of the sensitivity may be obtained by determining notched impact strengths over a range of different notch-tip radii. Its effect varies considerably from polymer to polymer, but PP is by no means the most sensitive of common thermoplastics to this phenomenon.